Brackets are a great way of adding character and charm to an addition or even a porch. To put in brackets that look right, you need to remember their original structural purpose, even if yours are just decorative.
Historically, the bracket was used either as a support or as a way to keep a timber frame building from racking. As such, the bracket is generally smaller than the beam and the column it supports. (Corbels are often the same size as the beam.)
Today, exposed overhead beams are generally used in a decorative way to imitate the structural exposed beams common in older construction. But designers who forget the beam's historical function as a structural member often place beams somewhat haphazardly.
Try to remember the beam's original purpose and different levels of finish when designing for modern spaces. For example, a rustic beam that's simple and has a rough, hand-hewn appearance — such as the beams found in many old farmhouses — conveys a rural feel. French manor houses had a more refined or dressed style, with overhead beams being hand planed and showing fewer adze marks. For better design, use the appropriate beam style to get the right sensibility.
In terms of placement, the smaller the beams, the closer the spacing: 4x4 beams might be 12 inches apart; 8x8 beams can be 18 to 24 inches apart. The rule of thumb is that the placement should make sense to the eye. — Brent Hull owns Hull Historical Millwork in Fort Worth, Texas, and is the author of Historic Millwork.